after about 3 to 4 seconds, the motor reaches itsrated speed and the current decreases, causing therelay contact to open and disconnect the winding.Current relays are ideal for use with split-phase,induction-run motors.Figure 14-35 is a schematic diagram of acurrent relay motor starting circuit.Voltage RelayA voltage relay looks much like a currentrelay; however, it differs in operation. It operateson increased voltage as the motor reaches ratedspeed, and, unlike the current relay, the contactsremain closed during the off cycle. When themotor is first turned on, it draws heavy currentand the voltage drop across the starting winding islow. As the motor picks up speed, there is less andless load; therefore, more and more voltage is in-duced into the winding. At about three-fourthsrated speed the voltage is high enough to cause therelay coil to pull the contacts open and disconnectthe winding. Voltage relays are used withFigure 14-35.Schematic diagram of a current relaymotor starting circuit.Figure 14-36.Schematic diagram of a voltage relaymotor starting circuit.capacitor-start motors. Figure 14-36 is a schematicdiagram of voltage relay motor starting circuit.Thermal RelayA thermal relay is commonly known as a hot-wire relay. It is available in at least two differentbasic designs and is supplied by several manufac-turers. All thermal relays operate on the theorythat electrical energy can be turned into heatenergy and that, when the temperature of a metalis increased, the metal expands. Thermal relays,like current and voltage relays, operate thestarting winding circuit. In addition, the thermalrelay controls the running winding circuit, if forany reason the circuit draws excessive current.The device consists of a specially calibratedwire made from a material with high oxidationresistance and two sets of contacts, all of whichare integrally attached to form the relay. Figure14-37 illustrates a typical thermal relay motorstarting circuit. The contacts are controlled by thehot wire, either through the use of heat-absorbingFigure 14-37.A typical thermal relay motor starting circuit.14-22